1. Field of the Invention
The present invention concerns a radio-frequency (RF) shield for an ultrasound transducer, or the type used in a magnetic resonance imaging or spectroscopy system.
2. Description of the Prior Art
When operating an ultrasound system such as a HIFU system, the ultrasound energy (ultrasound waves) must be transferred from the ultrasound transducer to the target body, such as a patient, via a suitable medium. It may also be advantageous for the medium to be a liquid that is capable of transporting heat away from the transducer or the body surface. Typically degassed water is used as such a medium.
Several problems can occur when operating such an ultrasound device in conjunction with a magnetic resonance imaging or spectroscopy system, as is the case in a magnetic resonance guided HIFU (MRg HIFU) procedure.
One problem is that RF antennas that are used for transmission in the magnetic resonance system may couple to the liquid or to the transducer itself. Such coupling may damage the ultrasound transducer, due to the high RF transmission powers that are used in magnetic resonance. Additionally, there will likely be energy (power) absorbed by the liquid or by the transducer, so that the RF transmission power for the magnetic resonance system has to be correspondingly increased. Moreover, the presence of the transducer and the liquid may cause the magnetic resonance transmission coil to become detuned, resulting in reflections toward the amplifier that is connected to the coil, and again requiring a higher transmission power. Lastly, the dielectric effect of the medium may adversely reduce the field homogeneity of the RF transmission field, thereby causing artifacts in the resulting image. In magnetic resonance systems that employ very high fields, this dielectric effect may cause specific absorption great hot spots, thereby causing discomfort, or even injury, to the patient.
Another category of problems results from the radio-frequency antennas of the magnetic resonance system that are used for reception coupling to the liquid or the transducer itself. This coupling of the reception antenna, due to losses in the liquid or in the transducer, may cause the magnetic resonance reception coil to detect (receive) a higher level of thermal noise. Moreover, the reception coil may become detuned, resulting in a higher noise factor in the amplifier connected to the reception coil, and thus a reduced sensitivity to the useful magnetic resonance signal that is being simultaneously detected.
Additionally, the electrical RF ultrasound signal that is sent to the ultrasound transducer (composed of one or more fundamental frequencies, their harmonics, and other frequencies that may exist in an “unclean” signal) may couple into the reception coil, and any portion of this coupled-in signal that is then digitized in the processing of the received MR signal will cause artifacts in the reconstructed image. The ultrasound signal may also possibly cause switching of diodes connected to the coil from the intended blocking state to a conducting state, thereby turning the coil off (precluding reception), or possibly causing the amplifier to saturate.
Another category of problems is due to the fact that the coupling medium may itself produce a magnetic resonance signal. It may be unavoidable for the field of view of the magnetic resonance system to be chosen so as to include a signal produced by the coupling medium, in order to avoid signal “fold-in,” and this may in turn increase the scanning time, or may reduce the achievable image resolution. Moreover, if the medium exhibits a flow, this may cause an artifact in the reconstructed magnetic resonance images.
Examples of such ultrasound systems are HIFU systems, non-focused therapeutic ultrasound lithotripsy, diagnostic ultrasound, ultrasound arrangements that induce shear waves for magnetic resonance-based shear-wave elastography, and other comparable ultrasound systems.
A number of techniques are known that address some of the problems noted above, but no technique is known that alleviates all of the above-cited problems.
Coupling media are known that do not produce contrast in a magnetic resonance image, such as oil, perfluorocarbon, etc. Assuming the medium that is employed does, in fact, not produce MR contrast, this approach may limit fold-in artifacts and flow artifacts. Such media, however, may still exhibit electrical loss and dielectric properties, which will define whether and how strongly the transmission and reception coils will couple to the medium. The use of such non-contrast-producing media, however, does not address the problem of coupling of the transmission coil to the ultrasound transducer, itself, and coupling between the ultrasound transducer to the magnetic resonance reception coil. Other factors that must be considered when choosing alternative liquids are cost, heat-carrying properties, ultrasound properties, aging properties, bio-compatibility and compatibility with other materials of the device.
Signal suppression techniques are also known, such as saturation bands and flow suppression, which reduce the signal originating from the ultrasound coupling medium. Such techniques, however, have not proven to sufficiently suppress the signal, and moreover have an impact on the signal acquisition. Moreover, because saturation bands are executed with a large flip angle, this further increases the local specific absorption ratio.
An ultrasound transducer has a ground plane, and therefore another known technique to alleviate coupling between the ultrasound transducer and the transmission and reception antennas is to segment the ground plane of the transducer. Such segmentation, however, is not always sufficient.
To alleviate the coupling between the electrical RF ultrasound signal and the reception coil, it is known to use blocking circuits that filter out unwanted signal contributions. Such blocking circuits are integrated into the reception coils, or their signal chain. This approach, therefore, requires the use of a specialized coil, and cannot be used in systems that are already fitted with conventional coils.
A further approach has been to synchronize the ultrasound sonication (activation) times so that magnetic resonance signal reception and ultrasound sonication do not occur simultaneously. This requires a synchronization circuit and architecture, and may limit the duty cycle of the ultrasound and/or of the magnetic resonance imaging. In the case of HIFU, for example, the available duty cycle is already significantly reduced when HIFU activation is interleaved with multi-slice magnetic resonance data acquisition. Using sequential acquisition of multiple slices enables a higher duty cycle of HIFU, but with the penalty of a lower signal-to-noise ratio in the magnetic resonance signal, and a lower temporal resolution.